1. Field of the Invention
The present invention relates to a radial borehole strain monitoring system for soft rock and, in particular, a fully self-contained and highly stable strain or deformation monitoring instrument of the vibrating wire strain meter type for monitoring radial borehole deformation in weak rock.
2. Prior Art
Strain meters of this type are well known and reference may be made to applicant's earlier Canadian Patent No. 1,240,851, issued Aug. 23, 1988. Those meters generally involve the use of a metal sleeve across the bore of which is strung a taut steel wire. The wire can be caused to vibrate by means of an electromagnetic transducer which is also housed within the sleeve. The frequency of vibration depends among other things upon the length and tightness of the wire and thus frequency can be measured by the same transducer which causes the wire to vibrate. In use, the meter is mounted securely within a hole bored within the rock formation the movement of which is to be monitored. Movement of the rock formation causes deformation of the sleeve which causes the tightness of the wire to increase or decrease and hence the vibration frequency to increase or decrease. The frequency which is displayed on a remote readout unit is, therefore, a measure of the strain.
Another form of known radial borehole deformation monitoring unit uses metal foil strain gauge transducers which are not recoverable once they are in place. Metal foil strain gauges suffer from problems of drift during long term monitoring because the epoxy bond is subject to shrinkage, moisture absorption, and creep from thermal expansion. In addition, the resistance of the read-out cable, varying with length, directly affects the signal output.
The use of vibrating wire transducer technology overcomes the disadvantages encountered when monitoring radial borehole deformation with metal foil strain gauges. Vibrating wire transducer signals are independent of read-out cable lengths and have demonstrated long term stability.